These hydrogenations are performed with selective catalysts, for example palladium on alumina.
Nevertheless the present catalytic processes are not entirely satisfactory; the yields, as a matter of fact, are often lower than 100% and the stability of the catalyst is generally insufficient.
These difficulties are illustrated hereinafter in relation with the hydrogenation of a C.sub.2 cut.
The ethylenic C.sub.2 cut, as obtained from a steamcracking unit, has generally the following composition:
acetylene--1 to 2% by volume PA1 ethylene--70 to 90% by volume PA1 ethane--10 to 30% by volume PA1 The exothermicity of the reaction requires the use of several reactors in series, since the gas phase is not favorable to heat removal. Moreover, the catalyst is used in large amount: a gas space velocity of 2 000 vol/vol/hour (corresponding to about 3 tons of catalyst for treating 10 t/h of C.sub.2 cut) is usual. PA1 The catalyst is not perfectly selective. The potential ethylene yield being 101 to 102%, the yields are hardly above 99.5%. Two factors are responsible for these low yields: an insufficient selectivity resulting in an excessive production of ethane and a parasitic polymerization of acetylene to more or less heavy products often called "green oils". PA1 These polymers settle on the catalyst and substantially reduce the length of the runs. PA1 better control of the reaction exothermicity, PA1 improvement of the hydrogenation selectivity and hence of the ethylene yield, PA1 improvement of the catalyst activity and stability.
The acetylene content of the ethylenic cut must be reduced to 2 ppm by volume, or less, by selective hydrogenation of acetylene over a palladium catalyst. This result has been obtained, up to now, by isothermal or adiabatic processes, mostly operating in gas phase under about 20 to 30 bars at a temperature usually ranging from 60.degree. to 150.degree. C.
These processes suffer from many disadvantages:
Processes involving the use of a solvent have already been described for the hydrogenation of acetylene. A process of this type is disclosed in U.S. Pat. No. 4,128,595 wherein the proposed solvent is an inert hydrocarbon. The use of a solvent offers several advantages as compared with the gas phase hydrogenation process, particularly the following advantages: